A torque transmission device selectably transmits drive torque provided by an internal combustion engine, for example, to a secondary axle, namely to a differential that acts between the wheels of the secondary axle. Such a torque transmission device can have a clutch—in particular a plate clutch—and a pump that responds to a speed difference between the primary axle and the secondary axle of the vehicle (for example, between an input element and an output element of the clutch). When such a speed difference arises, the pump creates a hydraulic pressure in a pressure chamber of the clutch (hereinafter referred to as the first pressure chamber). This pressure permits actuation of the clutch. A pressure rise in the first pressure chamber caused by a speed difference between the primary axle and the secondary axle thus effects engagement of the clutch and hence an increase in the portion of the drive torque transmitted to the secondary axle. Since the engagement of the secondary axle or the gradual locking of the clutch contributes to once again reducing the speed difference between the primary axle and the secondary axle, such an arrangement is essentially self-regulating in operation.
A torque transmission device may be hydraulically coupled to an actuating member to permit an at least partial locking of the differential of the secondary axle. In this way, the traction of the vehicle can be increased further by reducing a speed difference between the wheels of the secondary axle. The actuating member can likewise be a plate clutch. Associated with the actuating member is a pressure chamber, which is hereinafter referred to as the second pressure chamber. The first pressure chamber (which is to say the pressure chamber of the clutch of the torque transmission device) and the second pressure chamber (pressure chamber of the actuating member of the differential) are hydraulically coupled through a connecting line.
To be able to deactivate an automatic four-wheel drive, for example, in the event of intervention by a higher-level vehicle dynamics controller or for reasons of convenience, the first pressure chamber can be connected to a low-pressure chamber (e.g., a sump) through a discharge line. Located in this discharge line is a control valve that can be opened to reduce the pressure in the first pressure chamber (and, via the connecting line, ultimately in the second pressure chamber as well), even when a speed difference is present between the primary axle and the secondary axle. The second pressure chamber can also be connected to the low-pressure chamber through an additional discharge line to be able to reduce the pressure in the second pressure chamber and thereby release a prior locking of the differential.
EP 1,320,470 B1 (corresponding to U.S. Pat. No. 6,776,275 B2) describes a drive system of the aforementioned type.
While the prior art drive systems already permit satisfactory driving characteristics, it is desirable to further simplify the construction of such drive systems while also being able to establish various traction characteristics so as to be able to react appropriately to different driving conditions.